1. Field of the Invention
The present invention relates to series-resonant ballast circuits for gas discharge lamps.
2. Description of the Prior Art
A gas discharge lamp in the off condition is essentially an open circuit and requires a high voltage across its terminals to ionize or ignite the lamp. After the lamp is ignited, its effective resistance drops to a finite value, and its operating voltage drops to a fraction of the starting voltage. Because the lamp's effective resistance is a decreasing function of the RMS lamp current, some ballast impedance must be provided in series with the lamp to limit the lamp current.
A ballast circuit for operating an electric discharge lamp in accordance with the above requirements is disclosed in U.S. Pat. No. 4,053,813 to Kornrumpf et al. and U.S. Pat. No. 4,069,751 to Anderson. The disclosed ballast circuit comprises an inductor connected in series between the lamp and an AC power supply, and a capacitor connected in parallel with the lamp. The inductor and capacitor resonate at the power supply frequency or a harmonic thereof. When the lamp is off, the resonant circuit Q is high, and the lamp receives a high starting voltage proportional to the product of the Q and the power supply voltage. When the lamp ionizes or ignites, its effective resistance drops, thereby lowering the resonant circuit Q and causing the lamp voltage to drop to a lower operating value. The impedance of the series inductor exceeds the incremental negative resistance of the lamp and therefore limits the lamp current, as desired.
One disadvantage of the prior art ballast circuit as described in the cited patents is that the circuit Q is very low during lamp operation. The low Q implies that no substantial voltage step-up occurs, so that the power supply must supply a voltage comparable to the lamp operating voltage. If the lamp is of the type requiring an operating voltage of over 100 volts, then the power supply requires expensive high voltage transistors, or else may operate at a lower voltage and supply the lamp through a step-up transformer. Either the high voltage transistors or the step-up transformer undesirably increases the cost of the power supply.